Intravascular ultrasound (IVUS) imaging systems have been designed for use by interventional cardiologists in the diagnosis and treatment of cardiovascular and peripheral vascular disease. Such systems enhance the effectiveness of the diagnosis and treatment by providing important diagnostic information that is not available from conventional x-ray angiography. This information includes the location, amount, and composition of arteriosclerotic plaque and enables physicians to identify lesion characteristics, select an optimum course of treatment, position therapeutic devices and promptly assess the results of treatment.
Such IVUS systems generally include an IVUS device having one or more miniaturized transducers mounted on the distal portion of a catheter or guide wire to provide electronic signals to an external imaging system. The external imaging system produces an image of the lumen of the artery or other cavity into which the catheter is inserted, the tissue of the vessel, and/or the tissue surrounding the vessel. Problems encountered with these systems include clearly visualizing the tissue around the catheter, and identifying the precise location of the image with regard to known spatial references, such as angiographic references.
Before the development of less invasive approaches, the principal mode of treatment for occluded arteries was bypass surgery and, in the case of occlusions in the coronary arteries, coronary artery bypass surgery. Coronary artery bypass surgery is a highly invasive procedure in which the chest cavity is opened to expose the heart to provide direct surgical access to the coronary arteries. The procedure also includes the surgical removal of blood vessels from other locations in the patient's body (e.g., the sapheneous vein) which then are grafted surgically to the coronary arteries to bypass the occlusions. The recuperative period is lengthy with considerable discomfort to the patient.
The use of less invasive, catheter-based, intravascular techniques has developed for several decades and may be considered as the preferred mode of treatment for those patients amenable to such treatment. Typically, the intravascular procedures, such as angioplasty, atherectomy, and stenting require preliminary navigation of a guidewire through the patient's arteries to and through the occlusion. This guidewire, so placed, serves as a rail along which catheters can be advanced directly to and withdrawn from the target site. Total occlusions often cannot be treated with such minimally invasive intravascular approaches because of the inability to advance a guidewire through the stenosis. Typically patients with such occlusions have been treatable, if at all, by bypass surgery. Although in some instances, physicians may be able to force a guidewire through a total occlusion if the occluding material is relatively soft, attempts to force the guidewire through can present serious risks of perforating the artery. Arterial perforation can be life threatening.
The difficulties presented when trying to cross a total or near-total occlusion are compounded by the typical manner in which the anatomy of an occluded artery is diagnosed. Conventionally, such diagnosis involves an angiographic procedure in which a radiopaque contrast liquid is injected into the artery upstream of the occlusion and a radiographic image is made. The resulting image is that of the compromised lumen which necessarily differs from the natural arterial lumen. Although with angiographic visualization techniques, the physician can determine the location of the occluded region and an indication of the degree of obstruction, angiographic images do not provide a clear understanding of where, in the occluded region, the natural boundaries of the vessel are located.
As used herein, the term “severe occlusion” or “severe obstruction” is intended to include total occlusions as well as those occlusions and stenoses that are so restrictive as to require preliminary formation of a passage through the occlusion in order to receive additional intravascular therapeutic devices. Such occlusions have various causes and occur in both the arterial or venous systems. Total or near total occlusions occur in some instances as a consequence of the build-up of plaque or thrombus, the latter being problematic in arteries as well as in the venous system. For example, deep veined thrombus and thrombotic occlusion of vein grafts are serious conditions requiring treatment.
As noted above, recently techniques and systems have been developed to visualize the anatomy of vascular occlusions by using intravascular ultrasound (IVUS) imaging. IVUS techniques are catheter-based and provide a real-time sectional image of the arterial lumen and the arterial wall. An IVUS catheter includes one or more ultrasound transducers at the distal portion of the catheter by which images containing cross-sectional information of the artery under investigation can be determined. The ultrasound transducer(s) are typically spaced from the distal tip of the catheter. In that regard, the catheters typically include a distal tip formed of a radiopaque material such that the distal tip of the catheter is identifiable on fluoroscopy, x-ray, angiograph, or other similar imaging techniques. As a result of the distal tip, the ultrasound transducer(s) may be anywhere from one to five centimeters proximal of the distal tip of the catheter. For example, in each of the Atlantis SR Pro Imaging Catheter and iCross Coronary Imaging Catheter available from Boston Scientific Corporation, the ultrasound transducer is positioned 2.1 cm proximal of a marker band near the distal tip such that the ultrasound transducer is approximately 3 cm proximal of the distal tip of the device. Further, even in the EagleEye® Platinum RX Digital IVUS Catheter available from Volcano Corporation, the transducer array is spaced from the distal tip by a distance of 1 cm. This spacing of the ultrasound transducer(s) from the distal tip of the device is suitable for many vessel visualization applications and evaluations, but has limited effectiveness in the visualization and evaluation of severe occlusions
Accordingly, there remains a need for improved devices, systems, and methods for visualizing vessels having a severe blockage or other restriction to the flow of fluid through the vessel. In that regard, there remains a need for improved devices, systems, and methods for visualizing the severe blockage to facilitate safely crossing the blockage.